The present invention relates to a system for freshwater transportation and freshwater supply with the use of oil tankers, or the like.
Currently, the number of oil tankers sailing throughout the world is said to amount to approximately 4,000. Unlike freight ships, oil tankers (referred below to as ships) unload crude oil on buying countries and then return to petroleum producing countries while being empty. In this case, there is a fear that when ships are lightweight, hulls are bad balanced and safe sailing cannot be expected. Hereupon, in order to make hulls stable, ballast water (generally, sea water in crude oil buying countries is loaded and after returned to petroleum producing countries, the sea water is discarded) is loaded and ships return to ports.
However, marine pollution causes a serious problem over the world in recent years, and it is inevitable that a possible international treaty will inhibit sea water loaded as ballast water from being discarded unless being purified. Accordingly, there is a high possibility that ships cannot sail in the future unless they are provided with purifying apparatuses.
By the way, drinking water, to say nothing of agricultural water, industrial water, is short in petroleum producing countries, in which oil is drilled and supplied to the world. This is because the climate in petroleum producing countries is generally semi-dry or dry.
Hereupon, such petroleum producing countries manufacture fresh water by desalting sea water since natural water cannot meet the demand. In the present circumstances, the demand for water exceeds quantities of desalted water manufactured in these countries, and desalting is high in cost, so that it is not possible to readily increase production.
Incidentally, cost for desalting on land approximately amounts to 3.09 dollar/4550 liters (81.5 yen/ton: at the conversion rate of 120 yen to the dollar).
Cost for marine treatment approximately amounts to 1.59 dollar/4550 liters (41.9 yen/ton: at the conversion rate of 120 yen to the dollar).
In particular, the cost on land is as high as 81.5 yen/ton, and too expensive to be used for agricultural water and industrial water.
Accordingly, how inexpensively fresh water is supplied is an important problem.
Conventional techniques to solve the problem are disclosed in, for example, JP-A-51-143283 and JP-A-60-209382.
JP-A-51-143283 discloses a method of purifying treatment of fresh water loaded on a ship with the use of a purifying apparatus loaded on the ship, or of floatation separation of pollutant in fresh water after unloading of the fresh water and purifying treatment in a filtering apparatus.
JP-A-60-209382 discloses measures for accommodating plastic bags, which receive therein fresh water, in a hold to transport the same.
According to the disclosure of JP-A-51-143283 and JP-A-60-209382, purifying apparatuses and plastic bags are needed, which is very high in cost.
Hereupon, it is conceivable to load domestic wasted water, which is generated in crude oil buying countries, as ballast water to use the same as drinking water and agricultural water in petroleum producing countries.
Since the domestic wasted water (so-called foul water) contains a large amount of organic matter such as nitrogen and phosphorus, the water is drained to rivers and the sea after it is subjected to purifying treatment, in which nitrogen and phosphorus are reduced to predetermined reference values.
In this manner, there is a problem that purifying treatment for removal of nitrogen and phosphorus is costly in the present circumstances.
Incidentally, while it is found that nitrogen and phosphorus is effective for agricultural fertilizer, use thereof as organic resources is given up in Europe and Japan and so they are drained to rivers and the sea in the present circumstances because a large amount of food is imported from abroad.
Further, while organic fertilizer can be formed from sludge, which contains much organic matter such as nitrogen and phosphorus, sludge is subjected to incineration, in which expensive fuel is consumed, in the present circumstances because farmland having been used since old times has already retained organic matter, etc. adequately.
That is, sewage or sludge, which is effective for fertilizer, is made little use of.
It is an object of the invention to provide a freshwater supply system capable of making effective use of domestic wasted water, which has been drained as sewage to rivers or the sea.